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Bubble Fusion Furor
4 March 2002 (All day)
A whirlwind of controversy is swirling around one of the papers in this week's issue of Science. A team of scientists claims to have seen evidence for nuclear fusion in a beaker of organic solvent--a stunning claim that, if true, may eventually have important consequences for nuclear proliferation and energy production. But other scientists, citing an experiment that seems to belie the claim, are likening the paper to cold fusion.
Unlike nuclear fission, fusion is a very difficult process to get started. Only at extremely high pressures and temperatures will nuclei slam together forcefully enough to overcome their repulsion and merge. So difficult is it to achieve those pressures that hydrogen bombs use a small fission bomb to get the process going. Achieving a reasonable-sized fusion reaction on the tabletop has been out of reach. Researchers have tried--and failed stupendously, most notably in 1989, when premature claims of "cold fusion" in palladium wire humiliated the scientific community.
It is against this backdrop that nuclear engineer Rusi Taleyarkhan of Oak Ridge National Laboratory in Tennessee, nuclear engineer Richard Lahey of Rensselaer Polytechnic Institute in Troy, New York, and colleagues came forward with their claim of tabletop fusion. Their work relies on a phenomenon known as acoustic cavitation, in which sound waves rattling through a fluid create tiny bubbles and then cause them to expand and collapse. Starting with a small cylinder of acetone where all the hydrogens had been replaced with deuterium (a heavy breed of hydrogen that has an extra neutron), Taleyarkhan's team bombarded the cylinder with sound waves. At the same time, the researchers zapped the deuterated acetone with high-speed neutrons. The neutrons struck the acetone molecules, giving them an extra punch of energy and creating millimeter-sized bubbles--much larger than the bubbles created by the sound waves alone. The catastrophic collapse of these bubbles heats the deuterated acetone to the point at which deuterium atoms collide and fuse, the authors argue. The team claims to have detected neutrons flying out of the chamber and extra tritium in the vat--both signatures of fusion.
But the neutrons disappear when you use a more sophisticated neutron detector, claim Dan Shapira and Michael Saltmarsh, two physicists who are also at Oak Ridge who conducted a follow-up experiment to check Taleyarkhan's results. "There's no evidence for any neutron excess due to fusion," Saltmarsh says. Taleyarkhan and colleagues dispute Saltmarsh's interpretation. Other scientists take aim at other aspects of the experiment. "The paper's kind of a patchwork, technically, and each of the patches has a hole in it," says Mike Moran, a physicist at Lawrence Livermore National Laboratory in California.
More details about the contentious experiment and the controversy behind it will be available in this week's issue of Science.